This invention relates to a reception system having antenna diversity, for the reception of digital, MPSK-modulated satellite radio signals and/or terrestrially broadcast radio signals to a vehicle. The MPSK method (phase shift keying with M phase states) is particularly used for the radio transmission of digital signals in the frequency range above 1 GHz. Here, QPSK modulation (four phase states) is frequently selected for the downlink of a satellite radio connection, for energy reasons, because it has low sensitivity with regard to running time differences between superimposed signals, due to multi-path broadcasting. These statements relate to the basic form of this modulation method, as presented, for example, in “Digitale Mobilfunksysteme” [“Digital Mobile Radio Systems”], K. David/T. Benkner, Verlag B. G. Teubner, Stuttgart, 1996, page 158, under Chapter 4.2.3 Phasenumtastung [Phase Shifting]. This also holds true, for example, for the satellite radio system SDARS, which was designed for large-area mobile radio reception in the USA. In this connection, the same signal content is broadcast for radio broadcasting by two satellites in adjacent high-frequency bands of the same HF channel bandwidth B, but offset in time. Because of the uncertainty of the transmission from the satellite to the terrestrial receiver, particularly in vehicles, terrestrial radio broadcasting takes place in metropolitan areas, in addition to the broadcast from the two satellites.
It is known that the data stream of every digital signal transmission contains signals referred to as “burst signals” or “frame data” to synchronize the transmission. These signals are established according to the appropriate standard, and are repeatedly sent at time intervals during the frame period TR. The preamble signals are generally sent within a burst, along with other signals relating to the service, so that the time duration TB of the burst signal as well as the frequency of its broadcasting therefore have the effect of reducing the transmission of useful data, because the frame frequency FR=1/TR. Furthermore, the burst signal has symbols that contain the reference phase for phase synchronization of the system. In order to ensure reliable synchronization to the symbol cycle, even at high speeds, both the frame frequency FR, and the time duration TB of the burst signal must be selected to be suitably large.
The receiver-side detection of the carrier phases contained in the transmitted useful symbols can reliably take place in the receiver if the signal/noise ratio is sufficiently great, and the system is synchronized to the symbol cycle by means of the burst signals. If this were assured at every point in time, then it would be possible to operate without redundancy. However, a fundamental problem results from the fact that the carrier phase varies greatly over the travel distance s, in a reception field, in which interference exists due to multi-path broadcasting. Thus, these variations result in an incorrect detection of the symbols, for example if the error phase deviation is less than ±π/4 in the case of a 4PSK system. The phase progression of the reception signals is plotted as a multiple of π/4 of two antennas A1 and A2, over the travel distance s, of the vehicle, in multiples of the wavelength λ. The related level progressions of these antennas are compared to the phase progression of the received signals. The greater value of the two reception levels that occurs at the point in time in question is calculated as a heavy solid line. In the case of transmitter-side transmission of symbols having a phase that remains the same (e.g. in the phase state π/4), a high-frequency carrier would result as the reception signal when using one of the two antennas A1 and A2 are used on the vehicle, in each example, having the stochastic phase and amplitude progressions which are plotted in polar diagrams for the moving vehicle. Using a phase regulation that is available for this, according to the state of the art, for reception with only one antenna, the phase changes along the travel distance are therefore continuously regulated out. This occurs in operation, so that each of the symbols transmitted consecutively assumes one of the four intended phase states, and the deviation from this is adjusted from symbol to symbol. These phase regulation processes are susceptible to problems, and frequently lose the correct reference phase for correct detection of the symbols, particularly in the case of level loss, until another burst signal is transmitted for resynchronization. The phase progressions clearly show how the phases of the reception signals clearly differ and diverge even after only short travel distances. These phase regulation devices, which follow the reference phase for an antenna signal, have the disadvantage that when the antenna signal changes with antenna diversity switched on, and at any selected point in time, it loses the reference phase for this change, at first, and an extended resynchronization process takes place, necessarily resulting in an increased bit error rate.
The invention therefore provides a reception system having antenna diversity, which does not have the disadvantage of this phase regulation, and when the antenna signal changes, bit errors due to incorrectly detected useful symbols, are avoided.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose the embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.